The present invention relates to a method of applying motion-adaptive interpolation to a motion video signal e.g. a television signal to effect incremental operation of the number of frames such as televison scene by making use of motion information (motion vector) of a moving object. Specifically, the present invention relates to a method of effecting motion-adaptive interpolation by using a representative motion vector of which variation in respect to time lapse is reasonably limited. Further, the present invention relates to a device capable of producing an output such that a smooth-motion of a moving object is realized based on the motion-adaptive interpolation of missing frames.
Hitherto, in case where there is a need for compressing, to a great extent, a bit rate required for transmitting a motion video signal represented by a television signal, there has been employed a frame dropping scheme of transmitting frames of compressed volume obtained by dropping particular frames at a certain rate instead of transmission of all the frames. In this case, it is necessary to reproduce missing frames based on a synthesis operation by utilizing the transmitted frames on the side of a receiver. Namely, there arises a need for increasing the number of frames. As simple methods for meeting this need, there have been known a simple repetition method of repeating transmitted frames, and a linear interpolation method of effecting a linear operation based on picture signals spacially corresponding to frames adjacent to a missing frame etc.
However, the motion picture synthesized with the above-mentioned simple methods results in deteriorated picture quality such as, for example, lack in a smooth motion of a moving object (jerkiness), or degraded resolution in a moving area etc. To overcome such a deterioration, a motion-adaptive interpolation scheme has been proposed.
Various motion-adaptive interpolation systems have been developed. For instance, there is a system disclosed in ICC '84 LINKS FOR THE FUTURE IEEE International Conference on Communications May 14-17, 1984 Proceedings Volume 2 (pp. 527-1010). This system is such that a motion vector is obtained as unit of blocks each comprising a plurality of picture elements by using the frame adjacent to a missing frame, thus estimating a position of the moving object in the missing frame on the basis of the moving vector thus obtained. There is another system called a "gradient system" wherein a moving vector per units of picture elements is determined on the basis of a luminance gradient in each frame and a differential value between frames, thus estimating a position of a moving object in the missing frame based on the motion vector thus obtained. In addition, a rigid object motion approximation system has been developed with a view of lessening the occurrence of spacial discontinuity included within the interpolated picture, resulting from an erroneously-detected moving vector which does not correspond to an actual motion. For this purpose, the system is configured to remove an erroneously-detected motion vector among a group of motion vectors obtained with respect to a block or a picture element in a moving area by taking into account of an error per block with respect to various kinds of vectors, thereafter to estimate one motion vector (which will be called a "representative motion vector" hereinafter) which is representative of the motion of the moving area, thus shifting the entire moving area by using the representative motion vector, i.e., based on a rigid body motion approximation.
However, with the last-mentioned conventional system, when the number of motion vectors which do not correspond to an actual motion increases due to the influence of noises etc., the representative motion vector itself is erroneously estimated, with the result that there frequently occurs the case where the representative motion vector itself cannot correspond to the actual motion.
The drawbacks with this system will be described in detail in conjunction with FIGS. 1A and 1B. FIG. 1A shows an example of the case where the motion of a moving object is correctly estimated. Symbols F.sub.1 to F.sub.4 denote successive four frames wherein v.sub.1, v.sub.2 and v.sub.3 denote representative motion vectors estimated between F.sub.1 and F.sub.2, F.sub.2 and F.sub.3, and F.sub.3 and F.sub.4. In this example, the moving object moves with its velocity increasing. In contrast, if noises increase in a picture signal, there frequently takes place the case where a representative motion vector is estimated with a failure to correspond to the actual motion as shown in FIG. 1B, even when this system is applied to the same picture. At this time, there coexist a frame (e./g. F.sub.1) in which the representative motion vector is estimated in correspondence with the actual motion and a frame (e.g. F.sub.2) in which the representative motion vector is erroneously estimated without corresponding to the actual motion. As a result, when an attention is paid to a time axis direction, the velocity of the moving object which is to principally gradually vary in succession is varied in a discontinuous manner and to a great extent. When an attempt is made to newly synthesize a frame e.g. between F.sub.2 and F.sub.3, although the resultant picture can preserve plane continuity in a moving area, it has the drawback that its motion lacks in continuity such that the direction of the representative motion vector v.sub.2 is opposite to that of the correct representative motion vector e.g. in regard to F.sub.2, with the result that non-smoothed motion is likely to be reproduced.